This invention relates generally to optical devices, and more particularly to semiconductor laser devices.
Semiconductor lasers are used for generating light that carries data in fiber-optic systems. A common form of laser for long distance fiber optic communication is a distributed feedback (DFB) laser. In a DFB a diffraction grating is embedded in the laser and controls the wavelength. These devices can be made single mode and have narrow linewidths and excellent low noise characteristics appropriate for fiber optic applications.
Frequently, the intensity level from DFB lasers may require adjustment. This can occur for many reasons. For example, in wavelength division multiplexed (WDM) links, where light of many different wavelengths is traveling through a fiber simultaneously, one may need to adjust the optical power in each wavelength very carefully since erbium-doped fiber optic amplifiers used in fiber optic transmission systems tend to amplify stronger signals more than weaker signals. Typically, the maximum attenuation levels that may be required are between 3 and 10 dB. Unfortunately, merely decreasing the optical power exiting a DFB laser by reducing the current injected into the laser may affect optical properties of light exiting the laser. For instance, as the current is reduced in DFB lasers, the side-mode-suppression-ratio (SMSR) decreases, the linewidth gets worse, and the relaxation oscillation frequency moves to lower frequencies and increases the relative intensity noise (RIN).
Thus variable optical attenuators (VOAs) are useful and realized in many different embodiments. These include LiNbO modulators, liquid crystal devices, MEMs based mechanical shutters or mirrors, and thermo-optically activated glass waveguides. However, all these devices are separate optical components that are packaged either by splicing on to fibers, or by carefully aligning to the free space optical beam. This increases the cost and the complexity of the device.
One can also use a semiconductor optical amplifier (SOA) to adjust output power, and these devices can be integrated with the laser itself. In this case the SOA region is pumped quite hard to amplify the light of the laser. The light intensity can then be adjusted by controlling the current injected into the SOA. Unfortunately, the SOA, like any amplifier, also adds spontaneous emission noise to the light and degrades the optical signal. Amplifier regions are generally quite long, usually between 200 microns to 1 mm, and operate at high current levels of 150 mA to 500 mA.
Another device or alternative is an electro-absorption modulator (EAM). The structure is similar to the SOA, except that it is operated in reverse bias and the bandgap of the EAM is adjusted to be above the lasing energy, such that the device is transparent under normal conditions. As the device is reverse biased, the bandgap decreases due to Franz-Keldysh or Quantum Confined Stark Effect and the material becomes absorbing, reducing the power transmitted from the device. Generally these devices are used at high frequencies to encode the data on the optical signal and operated from zero volts to 2-3 volts reverse bias. Though the devices are usually quite short (<200 microns in length), the on-state loss is relatively high at 1-3 dB, and the fabrication is complicated since the bandgap is varied from the active region of the laser to the modulator region of the EAM.